Diabetes mellitus encompasses a group of syndromes whose main characteristic feature is the high blood-glucose levels, or hyperglycemia. This gives rise to a long-term series of vascular-type complications such as nephropathy, retinopathy, and neuropathy.
Two diabetes types are distinguished. The one known as type-I diabetes (IDDM), caused by the deficiency in the secretion of insulin, the regulating hormone of glucose in blood, and the type-II diabetes (NIDDM) that is characterized mainly by a resistance to the regulating effect of insulin, this latter type being so much or even more important than the first one.
The current treatments against type II diabetes, with sulfonylureas and biguanides, are far from being ideal. Sulfonylureas can induce hypoglycemia and biguanides can cause lactic acidosis.
In the last years, thiazolidinedione derivatives with hypoglycemic activity have been described and, therefore, with application in the treatment of diabetes, mainly of the so-called “type II diabetes”. The general structure of these compounds is: wherein R is a radical generally of aromatic or heteroaromatic type, and it can be linked or not to a N-type heteroatom.
A good review of this subject matter has been published by B. Hulin et al. (Current Pharmaceutical Design (1996), 2, 85–102).
As initial reference of this kind of structures the article of T. Sohda et al. (Chem. Pharm. Bull., 30 (10), 3580–3600 (1982)) may be taken into consideration. In this paper, the authors describe the synthesis of thiazolidinedione structures and, concretely, that of Ciglitazone, true starting point in the research of thiazolidinediones as hypoglycemic agents, also well-known as glitazones.
Several modifications have been made around the structure of these glitazones. 
Modifications have been introduced in the lipophyllic moiety of the Ciglitazone structure, which has lead to the synthesis of new such compounds as Troglitazone (EP139421), Rosiglitazone (EP306228), Pioglitazone (EP193256).
Furthermore, modifications on the ether function have been disclosed (D. A. Clark et al., J. Med. Chem., 1991, 34, 319–325; R. L. Dow et al., J. Med. Chem., 1991, 34, 1538–1544; B. Hulin et al., J. Med. Chem. 1992, 35, 1853–1864), in which this function is built-in in a cycle fused with the aromatic ring, such as it happens in Englitazone (EP207605B): 
Surprisingly, the authors of the present invention have found new thiazolidindiones that show very good hypoglycemic properties.